Soft start control method of LLC resonant converter

Abstract

The invention discloses a soft start control method for an LLC resonant converter, and the method employs a voltage outer loop + primary power inner loop scheme, and achieves the limiting of an output current and the prevention of the overhigh output current under the working conditions of on-load starting or lower output voltage or dynamic load through the control of the power. By controlling the primary side power, the control on the primary side current can be realized, so that the suppression on the current of the resonant cavity is realized, and the current of the resonant cavity is prevented from being too large; a frequency limiting curve is introduced, the minimum switching frequency is limited according to the output voltage, and when the output voltage rises, the minimum switching frequency is lowered. And by limiting the minimum switching frequency, the output gain of the LLC is suppressed, so that the output current is suppressed.

Description

Technical Field

[0001] The present invention relates to the technical field of switching converters, and in particular to a soft start control method for an LLC resonant converter. Background Art

[0002] LLC resonant converter is widely used in the power electronics industry due to its high efficiency, ability to achieve soft switching of primary and secondary switch tubes, and support for a wide range of input and output. At power-on startup, since the voltage of the output filter capacitor connected in parallel to the output end is approximately 0, the LLC resonant converter needs to charge the output capacitor, which will cause a large impact current. Therefore, a soft start solution is often introduced to reduce the impact current at startup.

[0003] The commonly used soft start solutions include:

[0004] 1) The duty cycle of the switch tube is maintained at 50%, and the switching frequency of the LLC resonant converter starts from a frequency point far higher than the resonant frequency, and then gradually reduces the switching frequency;

[0005] 2) Keep the switching frequency unchanged, and gradually increase the duty cycle of the switch tube from a small initial value to 50%;

[0006] 3) The switching frequency and the duty cycle of the switch tube are adjusted simultaneously. When the duty cycle of the switch tube increases gradually, the switching frequency decreases gradually.

[0007] In addition, the loop control scheme of LLC resonant converter commonly used in the industry is the outer voltage loop and inner current loop scheme. The voltage loop is used as the outer loop to control the output voltage, and the current loop is used as the inner loop to respond quickly and control the output current. The disadvantage of this scheme is that when the output voltage is suddenly lowered or the machine is started with load, the voltage outer loop is saturated and the resonant cavity current cannot be limited. Summary of the invention

[0008] The object of the present invention is to overcome the deficiencies of the prior art and provide a soft start control method for an LLC resonant converter.

[0009] To achieve the above object, the technical solution adopted by the present invention is: a soft start control method for an LLC resonant converter, comprising the following steps:

[0010] 1) Initialization stage: set the output current reference value I ref Initial value, loop parameter initialization, start emitting waves;

[0011] 2) Voltage soft start stage: output voltage reference value V ref Step by step, the output current reference value I ref The output capacitor is charged. When the output voltage reference value V ref Increase to the output voltage setting value Vset When , go to step 3);

[0012] 3) Current soft start stage: output current reference value I ref Step increase;

[0013] 4) While steps 2) and 3) are being performed, according to the output voltage V o Limit the minimum switching frequency f min , when the output voltage V o As the minimum switching frequency f min Gradually decreases, when the output voltage V o Increase to the threshold V o_tmp After that, the minimum switching frequency is reduced to the designed minimum operating frequency point f min_set ;

[0014] 5) When the output voltage reference value V ref and the output current reference value I ref After both increase to the set value and the output voltage reaches the set threshold, the soft start phase ends.

[0015] As a specific implementation method, during the soft start process of the LLC resonant converter, the voltage loop is used as the outer loop, the power loop is used as the inner loop, and the output voltage reference value V ref As the given value of the outer loop, the output voltage V o As the feedback value of the outer loop, the outer loop adopts PI control.

[0016] As a specific implementation method, the output current reference value I ref With the current output voltage V o Multiply them to get the current output voltage V o The maximum output power P Limit .

[0017] As a specific implementation method, the output V of the voltage loop is LoopOut With the maximum power P Limit Take the smaller one as the given value of the power loop, the bus voltage V Bus , primary current I Pri The product of the power conversion factor η serves as the feedback of the power loop.

[0018] As a specific implementation method, the output P of the power ring LoopOut Mapped to the switching frequency f, the output gain is adjusted by controlling the switching frequency f.

[0019] As a specific implementation, the output current reference value I ref for:

[0020]

[0021] In the formula, I init Indicates the initial value of the output current reference value, I set Indicates the final reference value of the output current, T 1 Indicates the current soft-start time.

[0022] As a specific implementation, the output voltage reference value V ref for:

[0023] V ref =V 0 +k*t

[0024] Where V o represents the output voltage, and k is a fixed constant for the output voltage step.

[0025] As a specific implementation, in step 4), the minimum switching frequency f min The calculation formula is as follows:

[0026]

[0027] In the formula, ε is the coefficient, V o is the output voltage.

[0028] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0029] 1) The present invention adopts a voltage outer loop + primary power inner loop solution to control the power to limit the output current when starting the machine with load or when the output voltage is low or the load is dynamic, so as to avoid excessive output current;

[0030] 2) By controlling the primary power, the primary current can be controlled, thereby suppressing the resonant cavity current and avoiding excessive resonant cavity current;

[0031] 3) Introduce a frequency limiting curve, which limits the minimum switching frequency according to the output voltage. When the output voltage rises, the minimum switching frequency is reduced. By limiting the minimum switching frequency, the output gain of the LLC is suppressed, thereby suppressing the output current. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 is the output voltage V o With the minimum switching frequency f min The relationship curve diagram of

[0033] Figure 2 A soft start control loop architecture diagram of the LLC resonant converter according to the present invention;

[0034] Figure 3 is the output of the power loopLoopOut Mapping relationship diagram with switching frequency f. DETAILED DESCRIPTION

[0035] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and specific embodiments.

[0036] The present invention provides a soft start control method for an LLC resonant converter, comprising the following steps:

[0037] 1) Initialization stage: set the output current reference value I ref Initial value, loop parameter initialization, start emitting waves;

[0038] 2) Voltage soft start stage: output voltage reference value V ref Step by step, the output current reference value I ref The output capacitor is charged. When the output voltage reference value V ref Increase to the output voltage setting value V set When , go to step 3);

[0039] 3) Current soft start stage: output current reference value I ref Step increase;

[0040] 4) While steps 2) and 3) are being performed, according to the output voltage V o Limit the minimum switching frequency f min , when the output voltage V o As the minimum switching frequency f min Gradually decreases, when the output voltage V o Increase to the threshold V o_tmp After that, the minimum switching frequency is reduced to the designed minimum operating frequency point f min_set , output voltage V o With the minimum switching frequency f min The relationship curve is as follows Figure 1 By setting the minimum switching frequency f min The limitation of can effectively limit the gain of LLC resonant converter and avoid excessive output current in the soft start phase.

[0041]

[0042] In the formula, ε is the coefficient, V o is the output voltage;

[0043] Here, f min With V o is a monotonic relationship, that is, V o Increase, f min The specific implementation form is not limited, as long as the monotonic relationship is guaranteed.

[0044] The method for obtaining the frequency limiting curve is as follows: within the output voltage range, a voltage detection point V is added every ΔV from 0. t , pay attention to the resonant cavity current I r When the output voltage V o Establish to the voltage detection point V t When the resonant cavity current I r Greater than the set protection threshold I r_th , then it is necessary to increase the minimum frequency f corresponding to this voltage point min , ensuring that the resonant cavity current does not flow and has a certain margin. Then monitor the next voltage detection point and repeat the above steps until all monitoring points have a suitable minimum frequency and the voltage during the startup process meets the set soft start curve.

[0045] 5) When the output voltage reference value V ref and the output current reference value I ref After both increase to the set value and the output voltage reaches the set threshold, the soft start phase ends.

[0046] Here, the output current reference value I ref for:

[0047]

[0048] In the formula, I init Indicates the initial value of the output current reference value, I set Indicates the final reference value of the output current, T 1 Indicates the current soft-start time.

[0049] Output voltage reference value V ref for:

[0050] V ref =V 0 +k*t

[0051] Where V o It indicates the current actual value of the module output voltage, and k is a fixed constant for the output voltage step.

[0052] In addition, during the soft start of the LLC resonant converter, the loop architecture is shown in Figure 2 As shown, the voltage loop is used as the outer loop and the power loop is used as the inner loop.

[0053] Specifically, the output voltage reference value V ref As the given value of the outer loop, the output voltage V o As the feedback value of the outer loop, the outer loop adopts PI control.

[0054] The reference value of the output current I ref With the current output voltage Vo Multiply them to get the current output voltage V o The maximum output power P Limit The power limit is related to the output voltage V o Strictly related, when the output voltage V o When P is small, Limit It is also small, which can control the secondary side to output a smaller power to avoid excessive output current at this time.

[0055] The P L imit has a minimum value P L imit_min, set the minimum value P L imit_min is to ensure that the output voltage can be effectively raised at the beginning stage under heavy load startup conditions to avoid failure to start the machine.

[0056] The output of the voltage loop V L oop O ut and maximum power P L imit is smaller and is used as the given value of the power loop. Bus voltage V Bus , primary current I Pri The product of the primary current I Pri As power feedback, it can effectively control the primary current to avoid excessive current in the resonant cavity, which may cause damage to the primary switch tube.

[0057] When the output voltage is relatively small, the voltage loop is saturated and the secondary power is limited by PLimit. At this time, the bus voltage has almost no fluctuation and the secondary current is limited by limiting the primary current.

[0058] The output of the power loop P LoopOut Mapped to the switching frequency f, P LoopOut The mapping relationship with f is as follows Figure 3 As shown in the figure, the output gain is adjusted by controlling the switching frequency f. min With f max Characterize the operating frequency range, f min is the minimum operating frequency, f max is the maximum operating frequency.

[0059] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A soft start control method for an LLC resonant converter, characterized in that: The following steps are involved: 1) Initialization stage: set the output current reference value I ref Initial value, loop parameter initialization, start emitting waves; 2) Voltage soft start stage: output voltage reference value V ref Step by step, the output current reference value I ref The output capacitor is charged. When the output voltage reference value V ref Increase to the output voltage setting value V set When , go to step 3); 3) Current soft start stage: output current reference value I ref Step increase; 4) While steps 2) and 3) are being performed, according to the output voltage V o Limit the minimum switching frequency f min , when the output voltage V o As the minimum switching frequency f min Gradually decreases, when the output voltage V o Increase to the threshold V o_tmp After that, the minimum switching frequency is reduced to the designed minimum operating frequency point f min_set ; 5) When the output voltage reference value V ref and the output current reference value I ref increases to the set value, and the output voltage V o When the set threshold is reached, the soft start phase ends.

2. The soft start control method of an LLC resonant converter according to claim 1, characterized in that: During the soft start process of LLC resonant converter, the voltage loop is used as the outer loop, the power loop is used as the inner loop, and the output voltage reference value V ref As the given value of the outer loop, the output voltage V o As the feedback value of the outer loop, the outer loop adopts PI control.

3. The soft start control method of an LLC resonant converter according to claim 2, characterized in that: The output current reference value I ref With the current output voltage V o Multiply them to get the current output voltage V o The maximum output power P Limit .

4. The soft start control method of an LLC resonant converter according to claim 3, characterized in that: The output of the voltage loop V LoopOut With the maximum power P Limit Take the smaller one as the given value of the power loop, the bus voltage V Bus , primary current I Pri The product of the power conversion factor η serves as the feedback of the power loop.

5. The soft start control method of an LLC resonant converter according to claim 4, characterized in that: The output of the power loop P LoopOut Mapped to the switching frequency f, the output gain is adjusted by controlling the switching frequency f.

6. The soft start control method of an LLC resonant converter according to claim 1, characterized in that: Output current reference value I ref for: In the formula, I init Indicates the initial value of the output current reference value, I set It indicates the final reference value of the output current, and T1 indicates the current soft-start time.

7. The soft start control method of an LLC resonant converter according to claim 1, characterized in that: Output voltage reference value V ref for: V ref =V0+k*t Where V o represents the output voltage, and k is a fixed constant for the output voltage step.

8. The soft start control method of an LLC resonant converter according to claim 1, characterized in that: In step 4), the minimum switching frequency f min The calculation formula is as follows: In the formula, ε is the coefficient, V o is the output voltage.

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